HD24/96 Technical Reference | 95 |
Basically, 29.97 fps time code is exactly the same as 30 fps time code except that it has been slowed down by about .1% so that it runs at the NTSC color video frame rate. The good news is that if you don’t produce anything that is going to synchronize to color NTSC video, you don’t need to use 29.97 fps time code.
Even though the invention of 29.97 fps time code solved the problem of synchronizing time code to color NTSC video, it created another problem: while the frame rate of the time code runs at 29.97 fps, it counts as though it is actually running faster. The net result is that at the end of an hour, the time code display reads 00:59:56:12 instead of 1:00:00:00. The problem is exactly analogous to why we have leap year – to correct for the fact that our calendar “runs fast” by about 0.07% (one day per four years). To correct this problem, drop frame code was invented. Drop frame code periodically “skips ahead” a bit to make up for lost time. The end result is that at the end of an hour, the slow clock has skipped ahead by 00:00:03:18 so that the clock reads 1:00:00 after an hour has passed. Drop frame works by skipping ahead (in count) by two frames every minute, except for the 10th minute, for a total of 108 frames per hour.
There are two drop frame codes which are used in the industry:
29.97 fps Drop | Used for broadcast NTSC video applications in the US, Canada and |
| Japan. |
30 fps Drop | Used primarily for correcting time code related problems in broadcast |
| NTSC video applications in the US, Canada and Japan. |
Drop frame time code is typically only used in broadcast applications where program scheduling is ruled by wall clock time (after all, you can’t broadcast the 12:00 news at 12:30). The great news is that unless you are a broadcaster or have to deliver something that will be broadcast on air, you don’t need to worry about drop frame time code either.
In practice there are actually four different speeds (rates) at which time code runs (24, 25, 29.97, and 30 fps), and four different ways that frames are counted (24, 25, 30, and 30 Drop). When you shake them all together and spill them on the table you get the six Time Code Frame Rates we described above. Whereas almost all equipment supports 29.97 Drop, not all equipment supports 30 Drop. The HDR24/96 supports all six standard fame rates, but don’t use 30 Drop unless you need to do so in order to be compatible with something else.
An important fact to know is that different manufacturers sometimes use different nomenclature for the same Time Code Frame Rates, primarily because of a failure to differentiate between the frame rate (how fast the code is running) and the frame count (how many frames in a second). This can be very confusing. The discrepancies always surround how 29.97 and 30 Drop and
As a final note, the MTC specification defines only 24, 25, 30, and 30 Drop frame rates (based on frame count). However, 29.97 and 29.97 Drop MTC can be generated simply by slowing down the rate of transmission slightly. Many devices, like the HDR24/96 do this. But because the Time Code Frame Rate is actually coded into the MTC data, an MTC reader (as might be found on a sequencer) might display that it is receiving 30 or 30 Drop frame time code when in actuality it is receiving 29.97 or 29.97 Drop frame. The only way to know what is actually happening is to know how your equipment specifically deals with these situations.